Field of the Invention
The present invention relates to driving control for motors, and in particular to driving control for a motor such as a stepping motor that can be used as a driving source of a load included in an image forming apparatus such as copiers or printers.
Description of the Related Art
In electrophotographic imaging forming apparatuses such as copiers and printers, stepping motors are widely used as a driving source of a conveyance system that conveys recording materials such as sheets of paper on which images are to be formed. A stepping motor is capable of performing speed control by controlling the period of a pulse signal that is provided to the motor, even if the motor is not provided with a mechanism for detecting the rotation speed of the rotor of the motor. Also, a stepping motor is capable of performing position control by controlling the number of pulses that are provided to the motor, even if the motor is not provided with a mechanism for detecting the rotational position of the rotor of the motor. These control methods are generally called “synchronization control”. However, if the load torque applied to the rotor of the stepping motor exceeds the output torque that corresponds to the driving current supplied to the windings of the motor during the synchronization control for the stepping motor, the stepping motor enters a step-out state in which the stepping motor is not in synchronization with input pulses and is uncontrollable. When the motor is in the step-out state, it is not possible to appropriately convey the recording materials and a paper jam may occur. Consequently, it becomes necessary to have a user remove the jammed paper from the inside of the image forming apparatus. In order to avoid such a situation, it is necessary to supply the windings of the motor with a current obtained by adding a predetermined margin to the driving current corresponding to the load torque required by the apparatus, so that the motor does not enter the step-out state. Consequently, there are problems in which the amount of power consumption increases, and motor noise increases due to excessive torque.
To address such problems, a method called “vector control” (or “field oriented control (FOC)”) has been proposed as disclosed in U.S. Pat. No. 6,850,027 and Japanese Patent Laid-Open No. 6-225595. Vector control is a method for controlling the amplitude and the phase of the driving current so that an appropriate torque is generated in the rotor of the motor, using a rotating coordinate system in which the direction of magnetic flux of the rotor is defined as a d-axis and the direction that is orthogonal to the aforementioned direction is defined as a q-axis. In the rotating coordinate system, the q-axis component (the q-axis current) of the driving current is a torque current component that causes the motor to generate torque, and the d-axis component (the d-axis current) of the driving current is an excitation current component that affects the magnetic flux strength of the rotor of the motor. Even if the load torque applied to the rotor of the motor changes, the motor control apparatus is capable of efficiently generating the torque required for the rotor to rotate, by controlling the q-axis current according to changes in the load torque. Consequently, it is possible to prevent the motor from entering the step-out state. It is also possible to prevent the power consumption from increasing, and to prevent motor noise from increasing due to excessive torque. In vector control, it is necessary to detect the rotational position of the rotor, using a position detection sensor such as a rotary encoder.
In the above-described vector control, the result of detection by the encoder, of the rotational position of the rotor, is used not only for rotor of the motor speed control and rotor of the motor position control, but also for controlling the driving current (the d-axis current and the q-axis current) in the rotating coordinate system. However, if the encoder becomes dirty due to toner used in the image forming apparatus or paper dust from recording paper for example, some sort of abnormality such as a missing pulse might occur in the pulse signal output from the encoder. If an abnormality occurs in the pulse signal, an error occurs in the rotational position of the rotor determined based on the pulse signal. If an error occurs in the determined rotational position of the rotor, the rotation speed of the motor becomes less stable, and there is the possibility of the motor entering an uncontrollable state.